It is now clear from a number of physiological and anatomical studies that the lateral geniculate nucleus is responsible for relaying "information" from distinct populations of retinal ganglion cells to different cellular layers in striate cortex. An important question which remains unanswered is how the information from these separate pathways contributes to the response properties of neurons which send their axons to other areas of the cortex and to the brainstem. One way to begin to understand this process is to study the morphology and connections of the neurons which lie within the main geniculo-recipient laminae of striate cortx (lamina 4) and to characterize them on the basis of dendriticmorphology, axonal configuration and neurotransmitter content. Unfortunately, this is difficult to accomplish since traditional Golgi methods do not generally impregnate myelinated axons in adult material and because methods which reveal the neurotransmitter affiliations of a neuron (for example, immunocytochemistry) most often label only the cell soma and proximal dendrites. The goal of the present application is to develop a tissue slice preparation for visual cortex that will allow us to: 1) intracellularly inject individual neurons in the main geniculate recipient laminae of striate cortex with horseradish peroxidase to determine their morphology and axonal connections, and 2) combine the intracellular injection technique with immunocytochemical methods in order to relate the morphology of individual neurons with their neurotransmitter content, and examine the distribution of immunocytochemically labeled terminals on the dendrites and soma of injected neurons. These experiments should provide valuable insights into the functional organization of visual cortex and may illustrate fundamental principles that apply to other regions of cerebral cortex as well.